Forensic workers transfer bodies from a van into a large truck in the northern border city of Matamoros, Mexico on Wednesday.

MEXICO CITY - Fifty-nine bodies were found buried Wednesday in a series of pits in the northern Mexico state of Tamaulipas, near the site where suspected drug gang members massacred 72 migrants last summer, officials said.

Security forces stumbled on the site as they were investigating reports that passengers had been pulled off several buses by gunmen in the area in what may have been an attempt at forced recruitment by a drug gang.

State and federal authorities conducted a raid that netted several suspected kidnappers and freed five kidnap victims.

Then they made a grisly discovery - a total of eight pits, containing a total of 59 corpses. One of the pits held 43 dead.

The Tamaulipas state government said the find was made Wednesday, and 11 suspects were detained, but the federal Interior department said the first pit was found Saturday and five suspects were detained by soldiers.

Tamaulipas state interior secretary Morelos Canseco said two of the dead were women. Many of the victims found in the pits appeared to have died between 10 and 15 days ago, dates that would roughly match the bus abductions, he said.

Canseco said state officials began getting reports that gunmen had been stopping buses, starting around March 25. At least two more cases were reported in the following days. The buses were allowed to continue on with their remaining passengers in each case.

The bodies were being examined to determine their identities and the causes of death, the Tamaulipas state government said in statement in which it "energetically condemned" the crimes.

The statement did not identify what drug gang, if any, that the arrested suspects belonged to, or why they might have hijacked the bus.

President Felipe Calderon's office issued a statement saying the find "underlines the cowardliness and total lack of scruples of the criminal organizations that cause violence in our country."

While there was no immediate confirmation that a drug cartel was involved, officials refer to the cartels as "criminal organizations."

The statement said Calderon had ordered federal officials to help in the investigation, and particularly in the work of identifying the victims.

The pits were found in the farm hamlet of La Joya in the township of San Fernando, in the same area where the bodies of 72 migrants, most from Central America, were found shot to death Aug. 24 at a ranch.

The area is about 80 miles (130 kilometers) from the border at Brownsville, Texas.

Authorities blamed that massacre on the Zetas drug gang, which is fighting its one-time allies in the Gulf cartel for control of the region.

The victims in the August massacre were illegal immigrants from El Salvador, Honduras, Guatemala, Ecuador and Brazil. An Ecuadorean and Honduran survived the attack, which Mexican authorities say occurred after the migrants refused to work for the cartel.

Mexican drug cartels have taken to recruiting migrants, common criminals and youths, Mexican authorities say.

It was unclear if the victims found Wednesday were migrants. Migrants frequently travel by bus in Mexico.

But drug gunmen also operate kidnapping rings, and erect roadblocks on highways in Tamaulipas and other northern states, where they hijack vehicles and rob and sometimes kill passengers.

Drug gangs across Mexico also sometimes use mass graves to dispose of the bodies of executed rivals.

The wave of drug-related killings - which has claimed more than 34,000 lives in the four years since the government launched an offensive against drug cartels - drew thousands of protesters into the streets of Mexico's capital and several other cities Wednesday in marches against violence.

Many of the protesters said the government offensive has stirred up the violence.

"We need to end this war, because it is a senseless war that the government started," said protester Alma Lilia Roura, 60, an art historian.

Several thousand people joined the demonstration in downtown Mexico City, chanting "No More Blood!" and "Not One More!" A similar number marched through the southern city of Cuernavaca.

Parents marched with toddlers, and protesters held up signs highlighting the disproportionate toll among the nation's youth. "Today a student, tomorrow a corpse," read one sign carried by demonstrators.